Light-polarizing process and product



June 30, 1959 Moleculury V. K- WALWORTH El AL LIGHT-POLARIZING PROCESS AND PRODUCT Filed July 2, 195a Orieni'ed Plush:

Confaining A Dye Mordun+ Righ+ Eye 'Rel ief Prinfing Mul'rix LeH' Eye Relief Prinflng Mufrix Y mvNToRs Z if a; a; 276m,

ATTORNEYS United States Patent LIGHT-POLARIZiN G PROCESS AND PRODUCT Vivian K. Walworth, Concord, and William H. Ryan, Billerica, Mass, assignors to Polaroid Corporation,

Cambridge, Mass, a corporation of Delaware Application July 2, 1956, Serial No. 595,212 28 Claims. (Cl. 88-65) This invention relates to processes for providing improved dichroic dye light-polarizing means characterized by having one or more dichroic dyes which are predeterminedly distributed in each light-polarizing layer of said means and held fast therein by a basic nitrogen-containing dye mordant present in each said layer, and especially relates to processes providing light-polarizing means of this nature in the form of image-bearing film which carries dichroic dye images, and further relates to the products produced by said processes.

Objects of the invention are to provide improved processing treatment whereby to raise the dichroism of dichroic dye light-polarizing means of the character having one or more light-polarizing layers of a transparent, molecularly oriented, linear, high molecular weight, hydroxyl-containing, vinyl polymer which each contains a dye mordant comprising a basic nitrogen-containing compound and which have each been dyed with a dichroic dye, by subjecting each said layer to the action of a solution of at least one electrolyte of the character containing at least one ion from the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series than barium or chloride; and further to provide processes of the character described for treatment of substantially uniformly dyed di chroic dye polarizers or for light-polarizing image-bearing films which may contain one or more monochromatic or multicolor dichroic dye images of a two-dimensional or stereoscopic nature.

Further objects of the invention are to provide improved processes for producing dichroic dye light-polarizing means of the character described as having one or more dyeable layers of oriented plastic material which each has a dye mordant of the character set forth dis persed therein and wherein each dyeable layer is predeterminedly dyed with at least one dichroic dye, following which each dyed layer is treated with a solution of at least one electrolyte of the character described; and to provide processes of this nature for producing a substantially uniformly dyed light-polarizing layer or one or more dichroic dye light-polarizing images of the char acter described in an image-bearing layer; as Well as to provide processes for producing dichroic dye lightpolarizing means wherein the polymer which provides each light-polarizing layer thereof is subjected to the action of a similar solution of at least one electrolyte at some stage prior to the predetermined dyeing of said layer, and wherein each said layer after being dyed is subjected to aftertreatment with said solution of said elecolyte.

A still further object is to provide light-polarizing products produced by processes of the character de scribed.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each of the others, and the products possessing the features, properties and the relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

Figure 1 is a diagrammatic illustration showing the manner of forming stereoscopic pairs of superposed lightpolarizing dye images in molecularly oriented film stock by the application to each side of the film stock of a printing matrix which contains, as a relief image, one of a stereoscopic pair of images to be printed and which is wet with a dichroic dye solution; and

Fig. 2 is a diagrammatic perspective view of a stereoscopic color print produced on the film stock by the procedure illustrated in Fig. l.

One practice for producing a light polarizer is to introduce a dichroic dye into a transparent, molecularly oriented, linear, high molecular weight, hydroxyl-containing, vinyl polymer of which a preferred example is polyvinyl alcohol. Wherever a dichroic dye is adsorbed in a molecularly oriented plastic sheet of this nature, it

renders the sheet light polarizing. Oriented plastic ma terials of the character described are useful to provide dye polarizers and are particularly suited as a medium for providing stereoscopic, light-polarizing transparencies and prints in color.

The present invention is concerned with techniques for providing improved light-polarizing products of the character heretofore set forth. To assist in the full understanding of the nature of these techniques and the improvements provided by their use, it seems desirable at the outset to point out the general nature of the products involved and the general manner by which dye polarizers are produced.

In this regard and specifically in connection with stereo scopic transparencies and color prints, a suitable medium or stock in which superposed, light-polarizing stereoscopa ic leftand right-eye images may be formed is providedby the film stock or printing blank 30 illustrated in Fig. 1 as comprising a laminar structure having two thin and transparent surface layers 31 and 32 mounted upon the opposite sides of a transparent support 33. The surface layers 31 and 32 are thin sheets of a transparent, molecularly oriented, high molecular weight, hydroxyl-containing, vinyl polymer of which a preferred example is polyvinyl alcohol, while the support 33 is any suitable transparent and substantially water-impermeable plastic such as cellulose acetate butyrate or cellulose triacetate.

Sheets 31 and 32 are prepared by conventional prac-- tices such, for example, as making a suitable casting composition of polyvinyl alcohol and into a sheet. Alternatively, the composition may be applied as a coating onto an already formed sheet of polyvinyl alcohol. Regardless are formed, they may be laminated by conventional means to the transparent support 33.

Preferably the layer 31 has its molecules so oriented that the transmission or polarizing axis of the layer will' be at 45 to the edge of the film stock 30, while the layer 32 has its molecules so oriented that the transmis-' sion or polarizing axis of the layer will also be at 45 to the edge of the film stock but will make an angle of with the transmission axis of the layer 31. This preferred orientation for film stock is indicated by the arrows 41 and 42 in Fig. 2 which schematically illustrates film stock 30 after image formation therein.

Film structures of this general nature are disclosed Patented June 30, 1959 casting this composition,

of how the sheets 31 and 32,

2,s92,sss A r V US. Patents Nos. 2,289,714, 2,289,715, 2,315,373 and 2,373,035. The film stock 30 provides a transparent structure useful as motion picture film or cut film. Additionally, the film stock 30 may be mounted upon an opaque, nonpolarizing, refiecting base and used for the formation of light-polarizing reflection prints. Stereoscopic image pairs formed in the film stock 30 will be located in superposed relation to each other.

One general practice for carrying out image formation in film stock 30 is effected with conventionally prepared gelatin washoff relief printing matrices which are each provided with one or more appropriate relief records to be reproduced in a layer of the film stock, such for example as red, green and blue separation records of a three-color image to be reproduced. Two sets of matrices are employed in stereoscopic reproduction of which a first set bears left-eye image records of each stereoscopic image pair to be printed in film 30, while the other set bears corresponding right-eye images.

The individual matrices of each set are dyed with a dichroic dye or dyes and are successively pressed into registered contact with the layer of the film stock 30 in which they are to reproduce an image by dye transfer. For three-color work, the red, green and blue printing matrices are respectively used to print cyan, magenta and yellow component images by means of dye solutions of these colors which are imbibed therein and may be printed in the order just named.

A convenient means for simultaneously printing on opposite sides of the film stock 30 is schematically illustrated in Fig. 1 wherein the film stock 30 with a righteye printing matrix and a left-eye printing matrix superposed on opposite sides thereof is shown as being passed between two rotating pressure-applying rolls 35 which cause the matrices to be pressed into intimate contact with the outer molecularly oriented layers 31 and 32 of the film stock.

-A transparency or product resulting from the utilization of the procedure illustrated in connection with Fig. 1 is diagrammatically shown in Fig. 2 wherein film stock 30 is illustrated as containing a right-eye stereoscopic image 310 in the molecularly oriented layer 31 and a left-eye stereoscopic image 320 in the molecularly oriented layer 32. Image 310 is shown in full lines while image 320 is shown in dotted lines. For purposes of simplification, the product shown in Fig. 2 has been considered as derived from a pair of stereoscopic records of the letter H and, in accordance with the process heretofore described, may be considered as providing a fullcolor stereoscopic transparency.

Viewing is carried out by observing these light-polarizing images 310 and 320 through polarizing filters located in front of each eye of the observer with the transmission axes thereof positioned at 90 to each other. Additionally, the light-polarizing filters are so positioned with respect to the light-polarizing images in the film 30 that each has its respective polarizing axis crossed or at 90 to the polarizing axis of the image to be observed therethrough. In this way, each eye sees substantially only the image intended for it.

It will be realized that the printing of light-polarizing images in the film stock of this invention is in no way restricted to the employment of gelatin washoff relief matrices. Any other procedure may be employed. As one alternative, the invention embraces the use of photomechanical printing plates or matrices such as halftone, line drawing and lithograph plates or matrices as means for printing images.

Photomechanical printing matrices may be used for printing dichroic dye images directly in undyed and oriented sheet stock which has been subjected to the pretreatment practices of this invention. As an alternative, a photomechanical printing matrix may be used to print a resist'area on 'oriented'sheet stock which has been pretreated and substantially uniformly dyedinaccordance with the practices set forth herein, following which the dye, in portions of the stock uncovered by the resist, is bleached. Upon the removal of the resist, the polarizing and bleached nonpolarizing areas may be used for positive and negative image formation or vice-versa.

Photomechanical printing is carried out by applying the matrix, after it has been wet or inked with a dichroic dye or a resist, into pressure contact with the sheet stock.

Procedures for forming dichroic dye images in oriented plastic sheet stock by the alternative practices described in connection with photomechanical printing are detailed in US. Patent No. 2,440,102.

As already noted, the inventive concepts of this invention embrace sheet polarizing means broadly. Thus, the layers 31 and 32 may be rendered light polarizing all over by substantially uniformly applying a dichroic dye to either or to both of said layers. Preferably, however, a sheet structure for conversion into a uniform density dye polarizer will comprise a transparent support such as the support 33 and an oriented layer such as the layer 31. Structures of this general nature and suitable for providing overall dichroic dye polarizers are disclosed in Patent No. 2,237,567.

Additionally, the film stock 30 may be employed to provide two-dimensional light-polarizing dye images by limiting image reproduction to one of the layers 31 or 32, although in such event it is generally preferable to employ a structure comprising the support 33 and only one image-bearing layer.

The molecular orientation of a plastic sheet is customarily carried out by stretching the sheet in the presence of heat and by'the application to the sheet of opposed tensional forces. As will be well understood, the result of such stretching is to cause orientation of the molecules of the sheet in a direction which is substantially parallel to the direction of application of the opposed stretching forces.

The degree of stretching imparted to a plastic sheet is conventionally measured by what is called the axial ratio which isa quantity determined by the ratio of the major axis to the minor axis of the ellipse appearing on the plastic sheet after the sheet is stretched and is derived as a result of the stretching of said sheet from a circle printed on the sheet before stretching. In general, the higher the axial ratio, the higher the efficiency of the sheet as a light polarizer. Axial ratios of 3 and greater are of a nature suitable for providing molecularly oriented sheet of high orientation. Current production practices are consistently carried out to provide axial ratios of around 6 for molecularly oriented polyvinyl alcohol sheet. The present invention is concerned with molecularly oriented plastic sheet stock which has an axial ratio of, or higher than, the value just noted.

One effect of stretching a plastic sheet, such as polyvinyl alcohol, is to cause the sheet to develop a resistance to dissolution in aqueous solution at given temperature. In general, this decreases in solubility of the plastic matcrial in aqueous solution is a function of the degree of its molecular orientation. Such decrease in the solubility of the plastic material affects the ability of an aqueous solution to permeate a sheet of the same and, at least to some extent, affects the introduction of the desired quantity of a dichroic dye solution into the sheet, whereby the resulting product may lack the desired dye density.

To overcome this difficulty, the present invention proposes to treat the material of the sheet or layer to be dyed with a solution comprising a special reagent or combination of reagents adapted to render the material more readily receptive to dye which is introduced therein in aqueous solution. This treatment is designed to promote dye transfer to a dyeable layer from a printing matrix or the introduction by other means of dye from an aqueous solution into a layer of dyeable material byincreasing the quantity'of dye removed from" the matrix or solu tion'and by'speeding"'up'the-rate'of'dye transfer or ini t'roduction. A result of this treatment is to provide products having a dye density greatly increased over untreated products of this character.

' The imbibition of an aqueous solution into a hydrophilic plastic material is most eflectively carried out when the material is completely or partially free of liquid which has been previously absorbed therein. It is therefore generally desirable to remove at least a part of the pretreating liquid penetrated into the plastic material before again subjecting the material to treatment with an aqueous solution such as a prewetting solution or dye solution. Such removal may be by either partial or complete drying of the plastic material. Although not always essential, in certain instances it is also desirable to at least partially remove the reagent or reagents of the pretreating solution present within the plastic material and in surface contact therewith. Such removal is effected before drying as by rinsing, for example with water, and may be followed by blowing excess liquid off the plastic material with an air knife or by means of a squeegee.

While such removal of a pretreating reagent is generally desirable, the need thereof is dependent upon a number of factors such as the reagent concentration of the pretreating solution, the presence in the pretreating solution of a reagent which will form surface scum or surface irregularities in or on the finished plastic sheet, the presence in the finished plastic sheet of a reagent which is adversely afiected by changes in the pH of the sheet brought about in the dyeing thereof, and the presence in dry sheet of a reagent capable of reacting with a solution which is used to wet the sheet prior to dyeing it as well as other and similar factors,

.This pretreatment is carried out by bringing the solution of the reagent or reagents into contact with the plastic sheet stock or the material thereof and is available for the treatment of organic plastic materials of the character described. The specific examples in regard to pretreatment and which are subsequently set forth have been directed to the treatment of preformed sheet materials with reagents of the general nature just discussed. In addition, the invention includes the pretreatment of plastic material comprising the sheet when said material is in the form of flake whereby to render the material more receptive to an aqueous dye solution, followed by the conventional formation of the flake into a sheet. When flake is subjected to pretreatment, the invention also contemplates the dyeing of the flake itself or the sheet formed thereof. In this regard, the specific practices of the illustrative examples of the invention suited for the pretreatment of plastic flake but, as will become apparent to the art, are subject to variation and simplification.

, The invention contemplates the pretreatment of sheet stock, for example preformed sheets of polyvinyl alcohol, which are in oriented or unoriented condition. in this regard, when overall or uniform density dye polarizers are to be made, the dyeing of a preformed sheet may take place prior or subsequent to the orientation of the sheet. However, where dichroic dye images are to be reproduced in sheet stock, it is generally preferable to orient the molecules of the image-bearing layers of the stock before the dyeing step is carried out.

In general, prints which are to be projected and magnified, i.e., transparencies for motion pictures, possess higher requirements as to maximum dye densities, image resolution and dichroisrn than reflection prints. Thus, in the 'case of stereoscopic color transparencies a maximum dye density of 2.7 is an acceptable minimum for projection purposes. By this invention, as will be outlined herein, densities of 3 and over have been achieved,

One method, already found effective for obtaining high dye densities in dye polarizing means, is to incorporate a basic nitrogen-containing mordant in the molecularly oriented plastic-sheet. Particular examples of suitable are generally mordants are described in the application of William H. Ryan and Vivian K. Walworth, Serial No. 444,074, filed July 19, 1954, which discloses the use of basic nitrogencontaining mordants incorporated in molecularly oriented hydroxyl-containing vinyl polymers. Other suitable mordants include the amino-containing polyethylenic compounds set forth in the application of Elkan R. Blout, William H. Ryan, Vivian K. Walworth and Howard C. Haas, Serial No. 526,301, filed August 3, 1955, and the basin nitrogen-containing polyamides set forth in the application of Howard C. Haas, Serial No. 526,302, filed August 3, 1955. The mordant may be introduced into the sheet material by including it in the casting composition from which the sheet is cast or otherwise formed. The sheet containing the mordant is then subjected to stretching to orient the molecules thereof. Alternatively, the mordant may be introduced into a preformed sheet, which has its molecules in oriented or unoriented condi tion, by the' imbibition into the sheet of an appropriate solution of the mordant.

A highly desirable result provided by pretreatment methods of the type described herein with respect to dyeing oriented plastic sheet, either alone or by combining these methods with a suitable mordant, is the limitation or restriction of lateral diffusion of a dye or dyes introduced to the sheet from a printing matrix so that the printed dye image formed in the oriented sheet or layer faithfully reproduces the fine detail of the matric image. The requirements relating to resolution of dye images are particularly exacting where projection prints such as slides or motion picture prints are contemplated. In' producing prints through methods of the present invention as, for example, in the production of stereoscopic image pairs of the invention, image resolution in excess of 60 lines per mm. is readily obtained.

While a mordant makes it possible to achive excellent image resolution in the formation of light-polarizing images, the presence of a mordant in molecularly oriented plastic sheets generally exerts an adverse effect on the dichroism of the final product. Inasmuch as the dichroism of a sheet of molecularly oriented plastic which has been dyed With a dichroic dye is a measure of its effectiveness to polarize light, it here seems desirable to set forth what is meant by the terms dichroic dye, dichroic and density ratio, which latter expression is a measure of the dichroism or effectiveness of the sheet.

In this regard, by a dichroic dye, there is meant a dye whose molecules possess the property of showing dichroism. In the practice of the invention, this property is displayed when said dye is incorporated in molecularly oriented plastic materials in that the resultant stained areas show dichroism. The term dichroism is used herein to mean the property of differential absorption of the components of an incident beam of light depending upon the vibration directions of said components. Thus, the optical density of each area of a light-polarizing image is a function of the vibration direction of light. incident thereon.

Light-polarizing images of the nature described are rendered not only in terms of density difference but in degree of light polarization and have low polarizing etficiency for light areas but high polarizing efliciency for dark areas. When a light-polarizing image is seen through a polarizer the axis of which is parallel to the polarizing axis of the image, it has extremely low contrast or may be totally invisible. Through a polarizer whose axis is at right angles to that of the light-polarizing image, the image is seen at its normal contrast. If two light-polariz ing images of a stereoscopic image pair are superposed with their axes at to each other and are looked at through a polarizer, ideally one image only will be visible at maximum contrast with the polarizer in one position, while the other image only will be visible when the polarizer has been rotated 90 from the first-mentioned posifion.

papers (see F. Hofmeister,

-If. an ideal light-polarizing sheet of the nature of layers 31 or 32 were who observed through an analyzer whose transmission axis is parallel to that of the polarizer, the density (now conventionally designated al would be zero. If the analyzer through which this ideal polarizer is viewed were rotated 90, the density along this axis (now conventionally designated 11 would be infinite. However, actual polarizers differ from this theoretical ideal. For example, in dye polarizers such as are used for the formation of light-polarizing color prints, the desired density 11 varies with wavelength and d is a smaller number which also varies with wavelength. The ratio d /d is called thedensity ratio, sometimes referred to as the dichroic ratio.

,It. is,not fully understood why a mordant incorporated in-a molecularly oriented sheet plastic such as polyvinyl alcohol generally exerts the previously described adverse efiect upon the dichroism of the dyed product. It has, however, been 'found as a part of this invention that the dichroism of molecularly oriented plastic sheet, such as polyvinyl alcohol which has a mordant distributed therein and which has been dyed with a dichroic dye, may be raised considerably if the dyed sheet is treated with one or more of the reagents heretofore described in connection with pretreatment. As will appear, a density ratio of Well over has been consistently achieved for dye images formed in oriented sheet plastics containing mordants by means of this aftertreatment. For stereoscopic photographic purposes, and especially in the case of projection, it is noted that a density ratio of 10 is an acceptable minimum.

'It is also pointed out that in the case of unmordanted oriented materials, the pretreatment procedures heretofore generally set forth and to be described in further detail provide, as an adjunct, an effective method for producing molecularly oriented and dyed sheets of satisfactory dichroism. In general, it may be added that pretreatment aids the attainable density ratio for unmordanted materials which are dyed with a dichroic dye or dyes. However, these effects of pretreatment in connection with unmordanted materials is not clearly understood.

The pretreating and aftertreating reagents with which this invention is concerned are electrolytes, including aqueous solutions of one or more acids, or bases or salts. By a solution of one or more electrolytes, there is meant an aqueous solution of one or more substances each of which dissociates in said solution to form ions having positive or negative charges. The present invention cornprehendsthe use of inorganic as well as organic substances of the nature just set forth. Similarly, by an electrolytic solution, there is meant a solution of one or more electrolytes.

The ability of the anions and/or cations present in a solution of an electrolyte to affect the characteristics or properties of hydrophilic colloids when in contact with a surface of the colloid, or when they have penetrated through said surface, is well recognized in the art. To give a few examples of treatment of this nature, it may be noted that one or more of the ions present in such a solution have beenemployed to affect the adsorption characteristics of surfaces in colloidal systems, to break up colloidal suspensions and to swell or dissolve hydrophilic polymeric material. In fact, the literature abounds with. disclosures of the action of ions in the treatment of a variety of different materials and shows that certain ions are more effective in their action than are others. With the aid of this knowledge, it has been recognized that ions. provided by electrolytes are classifiable in accordance withtheir effectiveness or activity in a recognized order or arrangement called the lyotropic series, and also frequently referred to as the Hofmeister series after the German scientist, F. Hofmeister, who set forth the efiectivenessof a wide number of electrolytes for various treatingpurposesin anurnber of individual scientific Arch. Exptl. Path. Pharmakol, volume 24, page 247 (1888); volume 25, page 1 (1888); volume .27, page 395 (1890); and volume 28, page 210 (1891).

The lyotropic series may mediately below:

C10 SO ,NO Cl, Br, P0 I, CNS, OH K, NH Na, Ba, Mg, Ca, Li, Zn, H

wherein the first or upper line of the series represents anions arranged in ascending order of activity or effectiveness from left to right, while the second and lower line of the series represents cations arranged in ascending order of activity or effectiveness, as a colloid swelling agent, from left to right. Thus, the anions C10 and OH represent the maximum differences in effectiveness between the anions of the series, while the cations K and H represent the maximum diiferences in effectiveness between the cations of the series. In the present invention, emphasis is directed to the activity of ions in the lyotropic series in relation to the ability of the ions to swell colloids or to render colloids more penetrable to a liquid or a solution applied thereto after pretreatment with a solution containing at least one ion of the lytropic series.

Where the lyotropic series appears in scientific literature, it is generally set forth in association with the explanation of some specific phenomena so that frequently the investigator or writer will include only ions which best illustrate his explanation. As a result, the series is not always set forth completely. The lyotropic series, as specifically set forth in the foregoing, is a compilation made from the following recent publications: Emil Ott, H. M. Spurlin and M. W. Graffiin, Cellulose and Cellulose Derivatives, part I, second edition, volume V of High Polymers, Interscience Publishers, Inc., New York, 1954-, and Ernest A. Hauser, Colloidal Phenomena, chapter 10, the Technology Press, M.I.T., Cambridge, Massachusetts, 1954.

The present invention is concerned with treatment by a solution of at least one electrolyte containing at least one ion, either an anion which has greater eifectiveness or activity than chloride or a cation which has a greater effectiveness or activity than barium in the lyotropic series. It may be noted that in carrying out the practices of this invention, it is unnecessary that both cations and anions of the just-noted order of eifectiveness be employed. The invention is successfully carried out where only one such ion results from the dissociation of the substance which provides the solution. It is also comprehended by the invention to utilize two or more ionizable substances at least one of which possesses one of the desired ions, while the other may or may not possess an ion that falls within the useful range of the lyotropic series.

As previously intimated, the term activity is employed herein to indicate the ability of an ion (cation or anion) toswell hydrophilic colloids or to increase their penetrability to liquids subsequently brought into contact therewith. By terms such as increasing activity, greater activity and the like, there is meant the ability of any ion in the lyotropic series to show increased effectiveness over any ion located below it in said series to swell a colloid as by causing said swelling to take place more rapidly and/ or to a greater degree. Such meaning of these terms in regard to activity is also intended to include the ability of any ion in the lyotropic series to show an increased efiectiveness over any ion located at a lower position in the series and the ability of the, ion of higher position to place a hydrophilic colloid in a condition where liquids, applied thereto after pretreatment with a solution containing said ion of higher position, will more rapidly penetrate the colloid or penetrate it to a greater degree than if the colloid had been treated with a solution containing an ion of lower position in, saidseries.

; Preferred examples of; substances which; are employed.

be expressed as shown im by the practices of this invention in the dyeing of molecularly oriented plastic sheets such as polyvinyl alcohol for the pretreatment of said sheets or the material thereof prior to dyeing and for the aftertreatment of the dyed sheets in instances where they contain a mordant are: zinc chloride, lithium chloride, calcium thiocyanate, sodium hydroxide and sulfuric acid.

Some examples of other substances which provide suitable solutions for pretreatment and aftertreatment purposes are as follows: sodium thiocyanate, lithium nitrate, potassium mercuric acid-chloride, sulfamic acid, triethanolamine, monoethanolamine, ammonium hydroxide, zinc salicylate, boric acid, ammonium thiocyanate, lithium iodide, calcium chloride, potassium iodide, trisodium phosphate, strontium iodide and sodium iodide.

As a few examples of solutions provided by two or more electrolytes, mention is made of calcium chloride and sodium thiocyanate; sodium hydroxide and sodium chloride; and sodium hydroxide and sodium carbonate.

In more detail, it is pointed out that all pretreating practices contemplated by this invention involve the processing steps of absorbing a pretreating solution into sheet stock or the plastic material thereof, followed by the partial or complete drying thereof, and may include the removal of at least a part of the pretreating reagent or reagents from the plastic material as, for example, by means of a water rinse carried out before drying. These practices, as will presently appear, are subject to variations within the specified limits, depending upon the concentration and character of the reagent or reagents of the pretreating solutions, the physical form of the plastic material operated upon, the time sequence of subsequent processing to convert the pretreated plastic material into one of the types of dichroic dye polarizing means heretofore mentioned, and other similar considerations.

The pretreatment of preformed sheet stock which is itself in oriented or unoriented condition is initiated by immersing the stock in the aqueous solution of the reagent, or by flowing said solution onto the stock or otherwise bringing the solution into contact therewith. Pretreatment of the material from which the sheet stock is to be subsequently formed, as for example polyvinyl alcohol flake, is carried out by wetting said material with the pretreating solution as by immersion therein preparatory to forming the flake into oriented sheet stock.

Treatment of the plastic material with a solution of one or more appropriate electrolytes may be carried out over a wide temperature range, i.e., from 32 F. to 120 F. and higher with aqueous solutions having a reagent concentration of from 0.5 to 30% and for time periods of from 30 seconds to five minutes.

In general, treatment time is decreased with increase in temperature and also by employment of a high concentration of the electrolyte or electrolytes employed, although the treatment time will be influenced by other factors such as the order of activity of the ions provided by the pretreating solution and the axial ratio of the sheet stock undergoing treatment in instances where oriented sheet is treated.

In instances where desirable, removal of the pretreating reagent or reagents from the sheet stock or the plastic material thereof is next carried out, it being here noted that such removal of the pretreating reagent or reagents is never objectionable except from the standpoint of adding additional processing procedure.

As an example of a condition where rinsing is unnecessary, consider the situation wherein pretreated sheet stock which is in a dry condition is to be preferentially dyed by means of an aqueous dye solution transferred thereto from a printing matrix brought into contact therewith. It is desirable to wet the sheet stock before pressing the printing matrix thereon and preferably to wet the stock with a solution of one or more reagents which improvedye transfer. If the reagent or reagents. of the pretreating solution are of a character which will not 'form' stantially to completion.

surface scum or surface irregularities on the pretreated sheet and also will not produce adverse effects during the dyeing of the sheet or by the prewet solution, it will be unnecessary to remove the pretreating reagent or reagents. A similar situation exists when the same reagent is used in a pretreating solution and in a prewetting solution and the reagent is one which produces no undesired effects. In addition, in some instances where the pretreating reagent is used in low concentration, deleterious elfects, which may be caused thereby, will be so minimized that it may also be possible to omit rinsing the pretreated plastic material prior to the drying thereof.

As specific examples of conditions where rinsing after pretreatment is unnecessary, mention is made of pretreatment of sheet stock with a solution of calcium thio4 cyanate and the subsequent prewetting of the sheet with r a solution of sodium acetate or with a calcium thiocyanate solution.

In general, neutralization of traces of an acid pretreatment solution is unneeded, as a neutral or acid environment is unobjectionable in carrying out the dyeing step. When pretreatment is carried out with a solution of a strong alkali such as sodium hydroxide, it is desirable to rinse out the pretreating solution and furthermore to' neutralize traces of the electrolytic reagent remaining in the sheet after rinsing to avoid undesired diflusion of dye When dye images are printed in the sheet stock with direct cotton dyes by means of relief printing matrices. In making uniformly dyed polarizers this problem does not occur.

Neutralization following pretreatment with sodium hydroxide is effected by contact of the sheet or the material thereof with an 0.5 to 1% solution of acetic acid for about one minute at room temperature. Following neu-' tralization, the sheet stock or the material thereof is again rinsed in water. In instances where neutralization treat ment is omitted, only one water rinse will be carried out.

As previously intimated, an important means for removing the pretreating solution from sheet stock or the plastic material thereof is by drying. For example, if the sheet. stock itself is being pretreated, the sheet may be dried by a warm air blast and if it has been subjected to rinsing prior to drying, rinse water may 'be blown off the sheet With an air knife or otherwise removed as by a squeegee. While removal of excess liquid from the surface of the sheet as by reagent in solution in said liquid, it will be realized that such treatment primarily serves to facilitate drying and can, of course, be omitted.

It has been previously pointed out that the formation of oriented sheet or film stock, such as that shown in Fig. 1, may take place either prior to pretreatment or after pretreatment. Generally in commercial practice it will be the oriented sheet stock itself that is subjected to pretreatment, followed by removal of at least a part of the pretreating solution from the stock.

With regard to the drying of the sheet stock or the plastic material thereof, it is to be observed that the completeness with which drying is carried out is not only dependent upon removing suificient liquid from the sheet stock or plastic material so that a dye solution may be readily adsorbed therein, but will also be dependent upon the time sequence of processing operations carried out after pretreatment to convert the sheet stock into the desired dichroic dye light-polarizing means. For example, if sheet stock, which is in oriented condition and which has been subjected to pretreatment, is to be wound into a roll and held in storage for an extended n'me prior to the dyeing thereof, it will be desirable to substantially completely dry the stock before winding it. On the other hand, if the dyeing operation is to be carried out at one step in continuous processing operations which are ini tiated with the pretreatment of the sheet stock, it will not be necessary to carry out the drying of the sheet sub- Similar requirements insofar as concerns drying and means of an air knife will also serve to remove.

1 1 also rinsing are applicable in regard to the pretreatment of plastic flake. For example, if the flake is to be dyed prior to being formed into sheet material, the extent of the drying may be somewhat limited, as is also the case in instances Where the pretreated flake without dyeing is immediately formed into a sheet.

Of course, in all instances whether preformed sheet material or the plastic flake from which the sheet is formed is being pretreated, it is generally unobjectionable, as pointed out, to subject the pretreated sheet or flake to rinsing followed by a drying operation which may be carried out substantially to completion.

It has been mentioned that oriented sheet stock which has been pretreated may be stored for extended periods prior to the dyeing thereof. This characteristic of the sheet stock is credited to the ability of the stock to retain the beneficial properties imparted thereto by pretreatment.

In fact, dichroic dye images of about the same desired degree of quality as regards maximum dye density, resolution and dichroisrn have been printed on oriented film stock which is of the character illustrated in Fig. 1 and which has either been-held in storage for several months after its pretreatment, or which is used immediately following pretreatment.

T better illustrate the invention, a number of specific practices relating to pretreatment procedure are set forth in the following examples. The film stock referred to in each of these examples is of the character of the film stock 30 of Fig.1 which includes layers in oriented condition and which is employable as motion picture film or cut film for the production of stereoscopic dichroic dye images. The individual steps in each specific example of pretreatment are numbered consecutively.

EXAMPLE ,I

l. Wetor-iented film stock two minutes with 20% sodium hydroxide at about 70 F.

. Rinse one minute in running water.

. Wet one minute with 1% acetic acid.

. Rinse four minutes in running water.

. Blow off excess; dry.

EXAMPLE II 1. Wet oriented film stock four minutes with 2% calcium thiocyanate solution at about 120 F.

2. Rinse one minute in running water.

3. Blow 01f excess; dry.

EXAMPLE III 1. Wet oriented film stock four minutes with zinc chloride solution at about 120 F.

. Rinse one minute in running water.

. Blow Off excess; dry.

EXAMPLE IV 1. Wet oriented film stock four minutes with lithium chloride solution at about 120 F.

. Rinse one minute in running water.

. Blow off excess; dry.

EXAMPLE V All of the foregoing examples have set forth a rinse step. The present example illustrates pretreating solution where rinsing is omitted.

EIQKMPLE VI A Other solutions suitable for carrying out pretreatnent procedure and used under conditions of time and temperakm ture as heretofore set forth in. general and as more specifically indicated in Examples I through IV are as follows:

Note that the neutralization step and the second rinse step of Example I are employed in instances where solution 3 in the just foregoing is used for pretreatment purposes. Furthermore, in regard to solution 3, sodium carbonate is employed therein because it may be advantageously employed to toughen an oriented polyvinyl alcohol film due to its ability to precipitate polyvinyl alcohol from aqueous solution.

The oriented sheet stock after pretreatment is ready for dyeing. When dichroic dye images are to be printed in oriented film stock, such as the film stock 30' of Fig. 1, it is generally desirable to wet the film stock, as the provision of film stock in a wet condition assures good overall contact between the surface being printed and the relief printing matrix pressed thereon.

Besides water as a wetting agent, mention may also be made of a 5% solution of sodium acetate or an aqueous solution of sodium benzoate and sodium sulfate wherein the sodium benzoate has a concentration of 1% and the sodium sulfate of 4% and other alkali metal salts of weak organic acids. Application of a prewetting solution is carried out by dipping the sheet therein for a period of from a few seconds to one or two minutes at temperatures ranging from room temperature up to F.,. after which excess solution is removed from the surface to be dyed preparatory to the dyeing step. Other prewetting solutions are disclosed in the copending joint applications of William H. Ryan and Vivian K. Walworth, Serial Nos. 431,341 and 431,396, both filed on May 21, 1954.

Following the just-described prewetting treatment, dichroic dye images are formed in the pretreated oriented film stock by appropriate direct cotton dyes transferred thereto from printing matrices which are pressed into contact with the film stock and which have aqueous dye solutions imbibed therein.

In instances where pretreated and oriented sheet stock is to be uniformly dyed to provide a uniform density dichroic dye polarizer, there are no problems as to printing matrix contact so that prewetting treatment prior to dyeing is generally unnecessary and dye imbibition may be carried out without this preliminary step.

In general, direct cotton dyes of an essentially elongated structure may be named as suitable for forming light-polarizing images in molecularly oriented plastic materials, and especially molecularly oriented polyvinyl alcohol. Dyes of this nature are set forth in the previously mentioned patents. As specific examples of suitable dyes, mention may be made of Niagara Sky Blue 6B (Cl. 518), Azoform Brilliant Blue G (Cl. 516) and Erie Fast Green CGB ((3.1. 589) for cyan; Solantine Red 8BL (C.I. 278), Solantine Pink 4131. (C1. 353) for magenta; and Solantine Yellow 4GL (Prototype 53), Stilbene Yellow 3GA (Cl. 622) and Solantine Orange 46 (Prototype 578) for yellow. Further reference to other suitable dyes appears in the examples which subsequently follow.

While the heretofore described pretreatment is usable with either mordanted or unmordanted oriented sheet stock, it is not always necessary to employ the pretreatment for mordanted oriented stock, especially in instances where the dye mordant provides an effective mechanism for obtaining the desired high maximum dye density for the dyed stock while speeding up the rate of dye transfer. Also as intimated herein, molecularly oriented sheet stock, which is free of any mordant and which has been predeterminedly dyed with a dichroic dye or dyes, is used as a light polarizer without any aftertreatment. n the other hand, an aftertreatment of molecularly oriented sheet stock which contains a mordant is generally desirable from the standpoint of raising the dichroism of the finished product.

This aftertreatment of mordanted film stock is carried out by contacting the dyed stock with a solution containing ions within the named range of the lyotropic series much in the same manner as the pretreatment of the undyed but oriented stock. A specific example of aftertreatment follows:

EXAMPLE VII Aftertreatment of molecularly oriented film stock, such as the stock 30 of Fig. 1, which contains a basic nitrogencontaining mordant and in which one or more dichroic dye images have been printed, is carried out in the following manner. The individual steps in this example of aftertreatment are numbered consecutively.

1. Wet the oriented and dyed film stock for a time period of from 30 seconds to one minute in a solution comprising 1% sodium hydroxide and 2 /z% sodium chloride at from room temperature to 120 F.

2. Rinse seconds in running water at room temperature.

3. Blow off excess; dry.

When sodium hydroxide solution is employed in aftertreatment, the neutralization step of Example I is generally unnecessary. Also, it is noted that it is unnecessary to subject unmordanted sheet stock to aftertreatment since the benefits derived thereby are substantially negligible.

It is generally preferable to rinse the printed stock following aftertreatment so as to remove aftertreatment reagents which may produce undesired etfects, such, for example, as by forming a scum on the surface of the printed film or by making the film stick. However, rinsing following aftertreatment is not always necessary as, for example, in instances where the concentration of the aftertreatment solution is sufficiently low or the reagent or reagents used therein are of a nature which will cause no deleterious effects.

As may be noted in Example VI, the aftertreatment solution possesses a reagent concentration which is considerably lower than the concentrations given for sodium hydroxide pretreatment solutions. A low concentration of active reagents in aftertreatment solutions is desirable so as to make it possible to easily remove them after the solution has carried out its function. At the same time it is to be understood that any of the pretreatment solutions named in Examples I through V are suitable for use as aftertreatment solutions, although it is preferable to employ a lower concentration of the reactive ingredient or ingredients named. In general, aftertreatment solutions employ an active reagent concentration of from about 0.5% to and are employed at temperatures ranging from about 70 F. to 120 F. Time periods for aftertreatment are influenced by the concentration of the treating solution and by temperature.

It is not known exactly why the pretreatments of this invention favorably influence the achievement of high orders of maximum dye density in dichroic dye polarizing products comprising one or more sheets of a molecularly oriented hydroxyl-containing polymer which may or may not have a mordant dispersed therein. Likewise, it is not entirely clear why these pretreatments favorably. afi'ect oriented sheet stock which is free of any mordant to permit the stock,on dyeing to be formed into a dichroic dyepolarizing product of a nature displaying exceptionally high dichroism. Similarly, it is difiicult to offer an exact explanation of why a mordant present in an oriented plastic layer tends to limit the order of dichroism achievable for the layer on dyeing it with a dichroic dye or why the dichroism of this layer may be raised by the aftertreatment practice disclosed herein.

That the benefits enumerated herein by the pretreatment and aftertreatment procedures do in fact occur is borne out by the following detailed examples which set forth a comparison of density ratios obtained in pretreated and unpretreated sheet stock and in aftertreated and unaftertreated stock as well as the effect of pretreatment on maximum density in pretreated stock and unpretreated stock. In each of these examples substantially similar film strips were processed, each strip comprising film stock of the nature of the film stock 30 of Fig. 1 and having oriented layers of polyvinyl alcohol 31 and 32, with each image-bearing layer having an axial ratio of 6 or more.

EXAMPLE VIII This example sets forth the effect of pretreatment on the density ratio of unmordanted oriented film stock.

The Erieform Violet 2R in Example VIII is a dye made by the National Aniline Division of Allied Chemical & Dye Corporation.

- EXAMPLE IX This example sets forth the effect of aftertreatment on the density ratio of mordanted and oriented film stock which has not undergone pretreatment.

Density ratio Pretreated N 0 pre- Dye by Extreatment am e I Chlorantlne Fast Red 5B (CI. 278) 18.6 14.4 Pyrazolone Orange 5G (Pr. 578) 13. 5 11. 2 Erieform Violet 2R 13. 4 11. 0

EXAMPLE X This example sets forth the effect of pretreatment on the maximum dye density of both mordanted and oriented film stock and unmordanted and oriented film stock.

Maximum dye density As has been previously stated, a mordant provides a means for readily obtaining a highly satisfactory reso lution for motion picture projection purposes. In fact monochromatic as well as multicolor stereoscopic dichroic dye images have been consistently produced by the practices of this invention which utilize a mordant to provide images having a resolution approaching 70 lines per mm. While it is difficult to obtain the desired minimum acceptable resolution'without the use of a mordant, it may be observed that monochromatic-andmulticolor dichroic dye images providing stereoscopic pairs'having a resolution closely approaching 60 lines per mm. have been consistently produced by the practices of this invention in oriented and unmordanted film stock. Stereoscopic dichroic dye images of a resolution lower than 60 lines per mm. are, of course, highly useful in the production of prints and transparencies which are to be viewed by hand and not projected.

Throughout the specification and claims, reference has been made to orientable, linear, high molecular weight, hydroxyl-containing, vinyl polymers and specifically to polyvinyl alcohol. Such reference will be understood to include polymers which have not been completely hydrolyzed, as for example partially hydrolyzed polyvinyl acetate, and is further intended to embrace polymers which may or may not have been subjected to stabilization by treatment with boric acid or other cross-linking agent, provided the polymeric material exhibits essentially the properties recognized by those skilled in the art as characteristic of commercially available polyvinyl alcohol.

Since certain changes may be made in the above product and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process for producing light-polarizing means of improved dichroism comprising the steps of forming into a film a transparent, linear, high molecular weight, hydroxyl-containing, vinyl polymer having distributed therein a basic nitrogen-containing dye mordant, orienting the molecules of said film, dyeing said polymerwith at least one solution of a dichroic direct cotton dye, and, after dyeing said film, absorbing thereinto a solution of at least one electrolyte,-the ions of which include at least one ion of the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series than barium and chloride, whereby the density ratio of said light-polarizing means is raised appreciably over that of light-polarizing means formed by a similar process excepting the step of treating said film with said electrolyte solution.

2. A process for producing light-polarizing means as defined in claim 1 wherein said film is subjected to a drying operation following the treatment thereof with said solution of said electrolyte.

3. A process for producing light-polarizing means as defined in claim 1 wherein said film is polyvinyl alcohol.

4. A process for producing light-polarizing means as defined in claim 1 wherein said solution of said electrolyte comprises sodium hydroxide.

5. A process for producing light-polarizing means as defined in claim 1 wherein said solution of said electrolyte comprises Zinc chloride.

6. A process for producing light-polarizing means as defined in claim 1 wherein said solution of said electrolyte comprises calcium thiocyanate.

7. A process for producing light-polarizing means as defined in claim 1 wherein said solution of said electrolyte comprises sulfuric acid.

8. A process for producing light-polarizing means as defined in claim 1 wherein said solution of said electrolyte comprises lithium chloride.

9. A process for producing stereoscopic pairs of dichroic dye images of improved dichroism, said process comprising the steps of forming two layers of a transparent, linear, high molecular weight, hydroxyl-containing, vinyi polymer having distributed substantially.uniformly therein a basic nitrogen-containing dye mordant,

orienting the molecules of each of said layers in a given direction, positioning said layers in superimposed relation whereby the polarizingdirectionof one of said layers is disposed approximately at 90 to that ofthe other layer, fastening said layers in fixed superimposed relation, printing an image in at least one ,color of one of a stereoscopic pair of images upon the first of said layers and, in proper registration, an image of the other of said stereoscopic pair of images upon thesecond of said layers by difierentially applying from individual image-bearing printing means a solution of at least one dichroic direct cotton dye to said layers, and, after the printing of said stereoscopic image pair, absorbing into each image-bearing layer a solution of at least one electrolyte, the ions of which include at least one ion of the class consisting of anions and cations selected fromthe lyotropic series and of greater activity insaid series than barium and chloride, the dichroism of said light-polarizing stereoscopic images being appreciably higher than that of images formed by a similar process, but which does not include said step of treating said image-bearing layers with sm'd electrolyte solution.

10. A process for producing dichroic dye light-polarizing images of the character defined in claim 9, wherein each image of said stereoscopicimagepair is a monochromatic dye image.

11. A process for. producing {dichroic dye light-polarizing images of thecharacter defined in claim 9, wherein each image of-said stereoscopic image pair is a multicolor dye image.

12. A process forproducing dichroic dye light-polarizing images of the character defined in claim 11, wherein each image of saidastereoscop c image pair is a multicolor image comprising cyan, magenta and yellow component images.

13. A process for producing dichroic dye light-polarizing images of the character defined in claim 9, wherein each'said layer comprises polyvinyl alcohol.

14. A process for producing light-polarizing means of improved dichroism having at least one molecularly oriented layer containing a dichroic dye so as to be rendered light-polarizing, said process comprising the steps of forming into a dyeable film a transparent, linear, high molecular weight, hydroxyl-containing, vinyl polymer having distributed thereina basic nitrogen-containing dye mordant, orienting the molecules of said film in one direction, laminating said'film toa supportnig sheet material, dyeing said film'with at least one solution of a dichroic direct cotton dye to render said film light-polarizing wherever dyed, and absorbing into said dyed film a solution of at least one electrolyte, the ions of which include at-least one ion of the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series-than barium and chloride, the density ratio of said light-polarizing means being substantially higher than that of light-polarizing means formed by a similar process, but which is devoid of said step of treating said film with said electrolyte solution.

15. A process for producing light-polarizing means as defined in claim 14 wherein the hydroxyl-containing vinyl polymer providing said dyeable film is additionally subjected to the action of a solution of at least one electrolyte absorbed intosaid polymer at a time prior to the dyeing of the layer, following which said polymer is at least partially dried, said solution of said electrolyte used in pretreating said layer prior to the dyeing thereof being substantially similar to said solution of said electrolyte absorbed into said layer after the dyeing thereof in that the ions of both said solutions of electrolyte include at least one'ion of the class consisting of anions and cations selected from the lyotropic series and of greateractivity in said series than barium and chloride.

16. In a process for producing dichroic dye images, the steps of providing, a layer of a dyeablematerial comprising a transparent, linear, high molecular weight, hydroxylcontaining, vinyl polymer which has the molecules thereof in a substantially. oriented condition and which con tainsa basic nitrogen-containing dye ,mordant therein,

printing said layer with et1 least-one dichroic dye image 17 by differentially absorbing into said layer a dichroic dye solution from a relief printing matrix which is pressed into contact with said layer, prior to printing said dichroic dye image in said layer improving the dye receptivity of the hydroxyl-containing vinyl polymer comprising said layer by subjecting said layer to the action of a solution of at least one electrolyte absorbed therein, and then at least partially drying said layer, and after the printing of said dichroic dye image again absorbing into said layer a solution of at least one electrolyte, said solution of said electrolyte used in pretreating said layer prior to printing said dichroic dye image being substantially similar to said solution of said electrolyte absorbed into said layer after the printing of said dichroic dye image in that the ions of both said solutions of electrolyte contain at least one ion from the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series than barium and chloride.

17. A process for producing dichroic dye images as defined in claim 16 wherein the molecules of said layer are in a substantially oriented condition at the time that said solution of electrolyte is absorbed therein prior to printing said dichroic dye image.

18. A process as defined in claim 9 wherein each said layer of a hydroxyl-containing vinyl polymer is additionally subjected to the action of at least one electrolyte absorbed into said polymer at a time prior to the printing of an image of said stereoscopic pair upon each of said layers, following which each of said layers is rinsed and dried, said solution of said electrolyte used in pretreating each said layer prior to the printing of a dichroic dye image thereupon being substantially similar to said solution of said electrolyte absorbed into each said layer after the dyeing thereof in that the ions of said electrolyte include at least one ion of the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series than barium and chloride.

19. A light-polarizing product showing a high density ratio comprising a film of a molecularly oriented, transparent, linear, high molecular weight, hydroxyl-containing, vinyl polymer, a basic nitrogen-containing dye mordant distributed substantially uniformly throughout said film, and at least one dichroic direct cotton dye carried by at least a portion of said film to render said portion light-polarizing, said film having had imbibed thereinto, after the dyeing thereof with said dye, a solution of at least one electrolyte, the ions of which include at least one ion of the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series than barium and chloride, whereby the density ratio of the light-polarizing portion of said product is substantially higher than that of a light polarizer formed, respectively, from a similar hydroxyl-containing vinyl polymer, a similar dye mordant and a similar dye but which has had no solution of an electrolyte imbibed thereinto.

20. A light-polarizing product as defined in claim 19, wherein said film is polyvinyl alcohol.

21. A light-polarizing product as defined in claiz: 19, wherein is included a pair of relatively fixed films of said molecularly oriented, transparent, linear, high molecular weight, hydroxyl-containing, vinyl polymer, each film including a substantially uniform distribution of said dye mordant, the molecules of one of said films being oriented in a direction substantially at 90 to the direc- 18 tion of molecular orientation of the other, each film having printed thereon one of a stereoscopic pair of lightpolarizing dye images, said images being printed in registered relation and in the medium of said dichroic direct cotton dye from a pair of individual printing means, said solution of an electrolyte having been iimbibed into said films after said dye images have been printed thereon.

22. A light-polarizing product as defined in claim 21, wherein each image of said stereoscopic image pair is a monochromatic dye image.

23. A light-polarizing product as defined in claim 21, wherein each image of said stereoscopic image pair is a multicolor dye image.

24. A light-polarizing product as defined in claim 23, wherein each image of said stereoscopic image pair is a multicolor image comprising cyan, magenta and yellow component images.

25. A light-polarizing product as defined in claim 21, wherein each said film comprises polyvinyl alcohol.

26. A light-polarizing product as defined in claim 19, wherein said film of vinyl polymer has additionally had absorbed thereinto, prior to the dyeing of said film, a solution of an electrolyte, following which said film has been dried, said electrolyte solution being similar to that imbibed into said layer after the dyeing thereof in that the ions of each of said electrolyte solutions include at least one ion of the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series than barium and chloride.

27. A light-polarizing product as defined in claim 19, wherein said dichroic direct cotton dye has been differentially absorbed into said film from a relief printing matrix to form a dichroic dye light-polarizing image in said film, and wherein said film has additionally had absorbed thereinto, prior to the printing of said image, a solution of an electrolyte, following which said layer has been at least partially dried, said electrolyte solution being similar to that imbibed into said layer after the dyeing thereof in that the ions of each of said electrolyte solutions include at least one ion of the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series than barium and chloride.

28. A light-polarizing product as defined in claim 21, wherein each of said pair of films has additionally had absorbed thereinto, prior to the printing thereon of one of said stereoscopic pair of light-polarizing dye images, a solution of an electrolyte, following which each said film has been rinsed and dried, said electrolyte solution being similar to that imbibed into said layer after the dyeing thereof in that the ions of each of said electrolyte solutions include at least one ion of the class consisting of anions and cations selected from the lyotropic series and of greater activity in said series than barium and chloride.

References Cited in the file of this patent UNITED STATES PATENTS 2,236,061 Izarc. Mar. 25, 1941 2,359,735 Kienle et a1. Oct. 10, 1944 2,373,035 Land Apr. 3, 1945 2,387,914 Kienninger Oct. 30, 1945 2,445,555 Binda July 20, 1948 2,454,515 Land Nov. 23, 1948 2,612,079 Mahler Sept. 30, 1952 UNITED STATES PATENT OFFICE V CERTIFICATE OF CORRECTION Patent No. 2,892,383 June 30 1959 Vivian K. Walworth et al.,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 14, EXAMPLE IX, in the table, second column thereof, for the heading, "Pretreated by Example I" read Aftertreated by Example VI Signed and sealed this 5th day of February 1963.

(SEAL) Attest:

ERNEST w. SWIDER DAVID LADD v Attesting Officer Commissioner of Patents I! j UNITED STATES PATENT OFFICE 1 CERTIFICATE OF CORRECTION Patent No. 2,892 383 June 30, 1959 Vivian Kn Walworth e1; alo

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 14, EXAMPLE IX, in the table, second column thereof, for the heading "Pretreated by Example I" read Aftertreated by Example VI Signed 'and sealed this 5th day of February 1963.

(SEAL) Attest:

RNE T w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents 

1. A PROCESS FOR PRODUCING LIGHT-POLARIZING MEANS OF IMPROVED DICHROISM COMPRISING THE STEPS OF FORMING INTO A FILM A TRANSPARENT, LINEAR, HIGH MOLECULAR WEIGHT, HYDROXYL-CONTAINING, VINYL POLYMER HAVING DISTRIBUTED THEREIN A BASIC NITROGEN-CONTAINING DYE MORDANT, ORIENTING THE MOLECULES OF SAID FILM,DYEING SAID POLYMER WITH AT LEAST ONE SOLUTION OF A DICHROIC DIRECT COTTON DYE, AND, AFTER DYEING SAID FILM, ABSORBING THEREINTO A SOLUTION OF AT LEAST ONE ELECTROLYTE, THE IONS OF WHICH INCLUDE AT LEAST ONE ION OF THE CLASS CONSISTING OF ANIONS AND CATIONS SELECTED FROM THE LYOTROPIC SERIES AND OF GREATER ACTIVITY IN SAID SERIES THAN BARIUM AND CHLORIDE, WHEREBY THE DENSITY RATIO OF SAID LIGHT-POLARIZING MEANS IS RAISED APPRECIABLY OVER THAT OF LIGHT-POLARIZING MEANS FORMED BY A SIMILAR PROCESS EXCEPTING THE STEP OF TREATING SAID FILM WITH SAID ELECTROLYTE SOLUTION. 